Piezoelectric materials have an effect of expanding and contracting when a voltage is applied (electrostrain effect) and reversely generating a voltage when pressure is applied (piezoelectric effect). Therefore they are a very important group of substances which can convert mechanical energy to electric energy. These materials have been widely applied to products in the field of energy conversion, such as actuators and sensors, and are widespread from daily life to cutting-edge technologies, playing an essential role in today's society.
Over the last five decades, PZT (Pb(Ti,Zr)O3 polycrystal) has been dominant in the most of the piezoelectric applications. These materials have the best piezoelectric properties: additive-free PZT has a maximum piezoelectric coefficient d33 of about 220 pC/N and soft PZT improved by addition has a maximum piezoelectric coefficient d33 of about 400 to 580 cp/N. Therefore they have been synonymous with piezoelectric materials in the past half century. However, since PZT contains a large amount of toxic lead, strict regulations have now been imposed on them from environmental reasons. Accordingly, developing a high performance lead-free piezoelectric material comparable to PZT is an urgent problem in the world.
Under such circumstances, there have been recently numerous studies on lead-free piezoelectric materials, but their piezoelectric and electrostrain properties are far inferior to those of PZT (most of the materials have a d33 of 200 pC/N or less). Although recently developed lead-free piezoelectric materials KNN-LT ((K,Na)NbO3—LiTaO3)) and KNN-LN ((K,Na)NbO3—LiNbO3) have a relatively good piezoelectric coefficient d33 of about 150-300 pC/N, the values are still less than that of soft PZT.
Moreover, KNN-LT and KNN-LN based materials have serious flaws of the difficulty in synthesis and high cost due to a large amount of expensive elements of Ta, La in the materials.
Non-Patent Document 1: Zhi Yu, Ruyan Guo, and A. S. Bhalla, Dielectric behavior of Ba(Ti1-xZrx)O3 single crystals. J. Appl. Phys. 88, 410 (2000)
Non-Patent Document 2: T. Mitsui and W. B. Westphal, Dielectric and X-Ray Studies of CaxBa1-xTiO3 and CaxSr1-xTiO3. Phys. Rev. 124, 1354-1359 (1961)